There are number of scientific and commercial applications for stable, single frequency laser output. An analysis of the energy levels of laser transitions would indicate that laser output would occur at discrete, well spaced intervals such that single frequency operation should be easy to achieve. However, in practice, a laser generates light over rather broad bandwidths referred to as the laser gain curve. This broadening occurs for a number of reasons. For example, in an ion laser, significant Doppler broadening occurs because the laser is operated at a very high plasma temperature. In addition, the magnetic fields used in an ion laser to contain the discharge creates Zeeman splitting which also broadens the line width. It is not uncommon to have a line width on the order of 10 GHz.
Since the laser cavity is a type of Fabry-Perot interferometer, the energy output over the gain curve is not continuous but occurs at discrete, closely space frequencies. The output frequencies are based upon the number of discrete longitudinal modes that are supported by the laser cavity. The longitudinal modes will occur at wavelengths such that an integral number of half wavelengths equals the length between the mirrors of the resonator. The calculated separation in frequency between these modes is quite small, on the order of 150 MHz.
Laser oscillation can occur only at frequencies where the gain curve exceeds the losses in the optical path of the resonator. In practice, the broadened laser gain curve exceeds the cavity losses over a large frequency range, on the order of 8 to 10 GHz. As noted above, there will be a number of discrete, closely spaced modes oscillating within this range.
Various approaches have been used in the prior art to limit the oscillation of a laser to one of the competing longitudinal modes. One of the most common methods requires the use of a frequency selective etalon.
There are number of types of etalons. In its simplest form, an etalon consists of a quartz glass plate with parallel surfaces that is placed in the laser resonator at a non-normal angle. Internal reflections give rise to interference effects which cause the etalon to behave as a frequency selective transmission filter, passing with minimum loss frequencies close to a transmission peak and rejecting by destructive interference other frequencies. In practice, the transmission peak of the etalon is set to coincide with a particular longitudinal mode resulting in single frequency operation of the laser.
The transmission peak of the latter type of etalon can be tuned by adjusting the angle of the etalon in the cavity or by varying the temperature. Tuning by adjustment of angle is somewhat limited since this approach tends to increase power losses. Therefore, the etalon is typically set at the minimum angle which will produce frequency selection while still maximizing power. The peak of the transmission curve of the etalon is adjusted by varying its temperature. In practice, the etalon is tuned so that its transmission peak is in alignment with a particular longitudinal mode and then held at a fixed temperature during operation.
Other types of more sophisticated etalons are available. One such type is described in U.S. Pat. Nos. 4,081,760 and 4,097,818, both assigned to the assignee of the subject invention and incorporated herein by reference. The etalons described in the these references consist of a pair of prisms having an air gap therebetween. A PZT crystal is attached to one of the prisms. When a drive voltage is applied to the PZT, the width of the air gap between the prisms is varied such that the transmission peak of the etalon is changed. As can be appreciated, the latter type of etalon is more expensive and more complex to operate than a solid etalon.
As noted above, the particular modes oscillating in a laser are directly related to the length of the resonator. Thus, as the length of the resonator drifts, the frequency of any given mode (and hence the frequency of the output of the laser) will also drift. As the frequency of the selected mode drifts, it will move out of alignment with the peak of the transmission curve of the etalon. As this occurs, the power output of the laser will drop as the mode drifts to lower and lower etalon transmission levels. If the length of the resonator continues to change, there will come a point where the next adjacent longitudinal mode will be transmitted by the etalon to a greater extent than the initially selected mode and the output of the laser will abruptly shift to the new frequency of the adjacent mode. This phenomena is called mode hopping. In practice, the laser power will often drop on the order of twenty percent between each mode hop.
There are many applications which can not tolerate mode hopping. The most sensitive application is in holography where exposures of photographic materials can take several minutes. Most holographic systems can tolerate some drift as the resonator length changes, but abrupt mode hopping can ruin processing.
The most straightforward method of minimizing mode hopping found in the prior art is to create a highly stabilized resonator where length changes are minimized. Length changes as small as 0.25 microns can cause a mode hop. Stability can be maximized if the laser optics are mounted to a SuperInvar rod that has a very low coefficient of thermal expansion.
The latter approach is useful if environmental factors can be kept relatively constant. For example, it is recommended that the temperature of the ambient air and water used to cool the laser be controlled. Unfortunately, these latter requirements are hard to meet. For example, in a typical 24 hour period, the ambient air temperature may rise and fall by four degrees C. Even with a SuperInvar support rod (expansion coefficient about 0.5.times.10.sup.-6 /.degree.C.), a temperature change of 0.5 degrees will cause a frequency shift of approximately 150 MHz and produce a mode hop. Thus, if the laser is run for 24 hours in a typical environment, 16 mode hops would occur.
Another approach for minimizing mode hopping is to actively stabilize the length of the resonator. In the latter approach, the position of the resonator mirrors are varied to maintain a selected resonator length even though the mounting structures are expanding or contracting due to temperature variations. Unfortunately, this approach is both complex and expensive.
Active stabilization systems are also found in tunable dye lasers. In a tunable dye laser, a plurality of components are used to scan the output of the laser over a large frequency range. These components typically include a birefringent filter, a tipping Brewster plate and one or more etalons. In such a laser system, a very fast and accurate method is needed to control the etalon so that the peak of the transmission curve follows the scanned frequency.
One type of active stabilization system is found in the Model 699 dye laser manufactured by Coherent, Inc. In this laser, an air gap etalon of the type described in the above cited patents is used. The transmission of the etalon can be rapidly adjusted by driving a PZT attached to one of the prisms. In order to more accurately control the system, the PZT drive voltage is dithered at a few KHz to induce a small amplitude modulation in the output of the laser beam. This amplitude modulation is phase-sensitively detected giving a discriminate signal permitting the peak of the transmission curve of the etalon to be locked to the cavity mode frequency. (See description in "Tunable Dye Lasers", T. F. Johnston, Jr., page 123, Encyclopedia of Physical Science and Technology, Vol. 14, Academic Press, 1987) While the latter approach is highly accurate, it is also complex and expensive.
Therefore it is an object of the subject invention to provide an improved system where the performance degradation associated with changes in the length of the resonator are addressed in a simple and inexpensive manner.
It is another object of the subject invention to provide a single frequency laser adapted for use with a passively stabilized laser resonator length.
It is a further object of the subject invention to provide a laser system where the etalon is actively stabilized without using an expensive air gap etalon or the associated dither and track circuitry.
It is another object of the subject invention to provide a laser system wherein the temperature of the etalon is actively controlled.
It is still a further object of the subject invention to provide a laser system wherein the variations in power are monitored in order to actively adjust the temperature of an etalon to prevent mode hopping.
It is still another object of the subject invention to provide a laser system with an output power stabilization loop wherein the variations in current are monitored in order to actively adjust the temperature of an etalon to prevent mode hopping.